The organic electroluminescent device is a self-luminous device and has been actively studied for their brighter, superior visibility and the ability to display clearer images in comparison with liquid crystal devices.
In an attempt to improve the device luminous efficiency, there have been developed devices that use phosphorescent materials to generate phosphorescence, specifically that make use of the emission from the triplet excitation state. According to the excitation state theory, phosphorescent materials are expected to greatly improve luminous efficiency as much as about four times that of the conventional fluorescence.
In 1993, M. A. Baldo et al. at Princeton University realized 8% external quantum efficiency with a phosphorescent device using an iridium complex.
Devices that use light emission caused by thermally activated delayed fluorescence (TADF) have also been developed. In 2011, Adachi et al. at Kyushu University, National University Corporation realized 5.3% external quantum efficiency with a device using a thermally activated delayed fluorescent material (refer to Non-Patent Document 1, for example).
In an organic electroluminescent device, carriers are injected from each of both electrodes, i.e., positive and negative electrodes to a light-emitting substance to generate a light-emitting substance in an excited state so as to emit light. It is generally said that in the case of a carrier injection type organic electroluminescent device, 25% of generated excitons are excited to an excited singlet state and the remaining 75% are excited to an excited triplet state. Accordingly, it is conceivable that utilization of light to be emitted from the excited triplet state, i.e., phosphorescence should provide higher energy use efficiency. However, in the phosphorescence, the excited triplet state has a long lifetime, and hence deactivation of energy occurs through saturation of an excited state and interactions with excitons in an excited triplet state, with the result that a high quantum yield is not obtained in many cases in general.
In view of the foregoing, an organic electroluminescent device utilizing a material which emits delayed fluorescence is conceivable. A certain kind of fluorescent substance emits fluorescence via intersystem crossing or the like leading to energy transition to an excited triplet state and the subsequent reverse intersystem crossing to an excited singlet state through triplet-triplet annihilation or thermal energy absorption. In the organic electroluminescent device, it is considered that the latter material which emits thermally activated delayed fluorescence is particularly useful. In this case, when a delayed fluorescent material is utilized in the organic electroluminescent device, excitons in an excited singlet state emit fluorescence as per normal. On the other hand, excitons in an excited triplet state absorb heat produced from a device and undergo intersystem crossing to an excited singlet to emit fluorescence. The fluorescence in this case is light emission from the excited singlet and hence is light emission at the same wavelength as fluorescence. However, the fluorescence has a longer lifetime of light to be emitted, i.e., a longer emission lifetime than those of normal fluorescence and phosphorescence by virtue of reverse intersystem crossing from an excited triplet state to an excited singlet state, and hence is observed as fluorescence delayed as compared to the normal fluorescence and phosphorescence. This can be defined as delayed fluorescence. Through the use of such thermally activated type exciton transfer mechanism, i.e., through thermal energy absorption after carrier injection, the ratio of a compound in an excited singlet state, which has usually been generated only at a ratio of 25%, can be increased to 25% or more. The use of a compound which emits intense fluorescence and delayed fluorescence even at a low temperature of less than 100° C. results in sufficient intersystem crossing from an excited triplet state to an excited singlet state by means of heat of a device, contributing to emission of delayed fluorescence. Thus, the luminous efficiency is drastically improved (refer to Patent Document 1 and Patent Document 2, for example).
Compounds of the following general formulae (X) and (XVI) having a 1,4-diazatriphenylene structure are proposed as a host material for a phosphorescent light-emitting material (refer to Patent Document 3, for example).

However, the above compounds are not disclosed as a light-emitting material attaining light emission of the compounds themselves. Emission of delayed fluorescence is neither disclosed nor suggested.